The elimination of crosstalk is an important objective in communications receivers. Basically, all frequencies outside of the narrow band of frequencies of interest are considered noise that degrades the signal information value of the communication. One type of bandpass filter in use today is the surface acoustic wave resonator filter which operates on the principles of piezoelectricity. The conventional prior art structure of a surface acoustic wave (SAW) filter comprises an insulative substrate sometimes having at least one conductively coated surface area ("earth pattern") upon which a surface acoustic wave element is mounted. The surface acoustic wave element comprises a surface acoustic wave propagating medium such as a piezoelectric substrate (e.g., LiNbO.sub.3, St-cut Quartz), an input electrode pair which consists usually of a pair of comb-shaped electrodes, both of which are mounted on the piezoelectric substrate, and an output electrode pair which also consist usually of a pair of comb-shaped electrodes both of which are mounted on the surface of the piezoelectric substrate. Each electrode pair is comprised of a plurality of elongated parallel electrically connected teeth or fingers with the fingers of one electrode interdigitized in predetermined spaced relation with the fingers of the other electrode. Such electrodes are usually formed of a conductive material, which may be vacuum deposited on a polished planar surface of the piezoelectric substrate. The conductive fingers cooperate as electrodes with the piezoelectric material thereunder, and each pair of interdigitized comb-shaped electrodes cooperate with the piezoelectric substrate to serve as an electroacoustic transducer. As an electrical signal is applied between the interdigitized electrodes of the input electrode pair transducer, surface waves are set up and propagate to the output electrode pair which becomes an output transducer which develops a voltage between its comb-shaped electrodes. In general, such devices have inherent frequency response centered at a frequency determined by the center-to-center spacing of the interdigitized fingers of each of the transducers. For example, in a television IF embodiment disclosed in U.S. Pat. No. 3,573,673DeVries et al, utilizing PZT as the piezoelectric substrate, the fingers of both the input transducer and the output transducer are approximately 0.5 mil wide and are separated by 0.5 mil for the application of an IF signal in the typical range of 40 to 46 megahertz. The spacing between the input transducer and the output transducer is approximately 60 mils and the width of the wavefront is approximately 0.1 inch. This structure can be compared to a cascade of two tuned circuits the resonant frequency of which is determined largely by the spacing of the fingers.
Crosstalk results when a signal is directly transmitted from the input transducer to the output transducer without the time delay required for a wave introduced by the input transducer to reach the output transducer. As a result, two sets of signals from the input are transmitted to the output, one of which is delayed in time relative to the other, resulting in crosstalk. Crosstalk is partially caused by capacitive coupling between the input and output transducers. Reducing such coupling by increasing the spacing between transducers is undesirable due to the resultant increased wave attenuation associated with the larger wave path distance.
Another source of crosstalk is bulk waves produced concurrently in the piezeoelectric with the desired surface waves. Such bulk waves travel through the piezoelectric at a different velocity and through a different path than the surface wave. It is known in the prior art that bulk wave effects can be reduced by increasing the thickness of the piezoelectric substrate. Also known is the fact that filter selectivity can be increased by increasing the number of teeth in the combs of the electrodes. However, this also increases capacitive coupling as does increasing the width of the fingers of each comb. U.S. Pat. No. 3,573,673, DeVries et al, discloses the use of maximal distance separation such that the input electrode having signal content is closer to the output ground electrode than the output signal electrode, since the potential induced on one electrode by another is reduced as the spacing is increased, and also since field intensity decreases as the distance between electrodes increases. The signal electrodes are maximally spaced apart while the ground electrodes of input and output are closely spaced. In addition, vertical wall shields between input and output electrode pairs of a single SAW filter are disclosed.
In U.S. Pat. No. 4,126,839, Yamanoi et al, an earth ground pattern is disclosed as separated between the signal input and signal output points in order to reduce the cause of the inductive and capacitive couplings between the signal input and signal output of a SAW filter. Wire leads connect the ground electrodes of the input and output to the earth pattern.
In order to obtain the selectivity required for many communication products, it is necessary to use at least two SAW resonator filters. Coupling occurs between input and output leads in many configurations which allows unwanted frequencies to pass, thereby degrading filter performance. Conventional solutions to this problem involve packaging the filter separately which is presently expensive and bulky. At high frequencies the inductance of leads to a ground plane becomes significant and the floating ground plane can carry the undesired signal from the input to the output, thereby degrading the effectiveness of the SAW filter.
The use of bent wire bonds to connect the printed conductive patterns on the ceramic substrate to the surface of a piezoelectric for the purpose of reducing temperature effects or corresponding frequency response and allowing crystal expansion and contraction is well known, as disclosed for example, in U.S. Pat. No. 3,828,210, Livenick.